(a) Field of the Invention
The present invention relates to a tracking error detecting unit and, more particularly, to a tracking error detecting unit for use in an optical disk drive for driving an optical disk file.
(b) Description of the Related Art
A conventional tracking error detecting unit in a head tracking system of an optical head will be described with reference to FIGS. 1 to 4. The detecting unit for an optical head uses three beams for obtaining a tracking error signal.
FIG. 1 shows a block diagram of the tracking error detecting unit 50. The tracking error detecting unit 50 includes a diffraction grating 51 for diffracting a beam emitted from a light source 101 of the head tracking system 100 to generate first-order diffracted beams, a refocusing lens 53 for refocusing reflected beams from a beam splitter 52, a photodetector 54 for receiving the reflected beams, which have been refocused through the refocusing lens 53, to output electric detected signals, and a tracking error calculating section 58 for calculating a tracking error signal based on the detected signals.
In the head tracking system 100 of FIG. 1, there is also provided a collimating lens 102 for collimating the beam emitted from the light source 101, and an objective lens 103 for focusing diffracted beams generated by the diffraction grating 51 onto an optical disk file 70.
FIG. 2 shows the beams focused on the optical disk file 70, and FIG. 3 shows the beams received by the photodetector 54. In FIG. 2, reference numeral 105 denotes a focused spot of a main beam, i.e. the zero-order diffracted beam, and reference numerals 106 and 107 denote focused spots of sub-beams, i.e., a pair of first-order diffracted beams, which are positioned slightly away from each other in the radial direction (P) of the disk file. Those beams pass through the objective lens 103 to refocus on the optical disk file to form optical spots.
The optical spots 105,106 and 107 are formed in the following manner. In FIG. 1, the beam emitted from the light source 101 passes through the diffraction grating 51 so that the beam is split into a zero-order diffracted beam and a pair of first-order diffracted beams. These beams are reflected by the optical disk file 70 and travel along their original paths, and then part of each beam is reflected by the beam splitter 52. The reflected beams pass through the refocusing lens 53 to focus on respective detecting sections 55, 56 and 57 of the photodetector 54, as shown in FIG. 3.
The photodetector 54 includes a plurality of detecting sections 55, 56, and 57 for receiving a zero-order diffracted beam and two first-order diffracted beams reflected from the optical disk file 70 to output electric detected signals. The two first-order diffracted beams 106 and 107 are located on both sides of the zero-order diffracted beam 105. Detected signals corresponding to the first-order diffracted beams 106 and 107 are input to the tracking error calculating section 58 of FIG. 1.
The reflected main beam 105 and the reflected subbeams 106 and 107 detected by the detecting sections 55, 56, and 57, generate output signals, i.e., detected signals. The detected signal S105 corresponding to the reflected main beam 105 is used for generating a reproduced signal and a focus error signal, and the detected signals S106 and S107 corresponding to the reflected sub-beams 106 and 107 are input to the tracking error calculating section 58.
The tracking error calculating section 58 is implemented by an electrical circuit, which calculates a tracking error signal based on the difference between the detected signals representing the first-order diffracted beams 106 and 107 in accordance with a program previously input, and which outputs the tracking error signal.
FIG. 2 illustrates the locational relationship between tracks of the optical disk file 70 and a plurality of the optical spots 105, 106 and 107 focused on the optical disk file 70. In this example, each track includes a land portion 71 on which data pits, for example, are formed, and a groove 72 which is used for tracking operation.
FIG. 4 shows waveforms of the detected signals S106 and S107 output from the photodetector 54 as functions of the radial direction of the disk file The tracking error calculating section 58 calculates the difference between the detected signals S106 and S107 to obtain a tracking error signal "G" based on the difference. The calculation of the tracking error signal "G" on the basis of the subbeams 106 and 107 is based on the principle described below.
A minimal point of the signal waveform of the subbeam 106 corresponds to a state in which the subbeam 106 is irradiated onto a land portion 71 of the disk file and in which the signal amplitude is minimized due to diffraction caused by the land portion 71. A maximal point of the signal waveform corresponds to a state in which the subbeam 106 is irradiated onto a groove 72 and in which the signal amplitude is maximized. Thus, both the distance between the minimal points and the distance between the maximal points correspond to the track pitch Tp.
Since the subbeam 107 is offset slightly from the subbeam 106 in the radial direction (P) of the optical disk file 70 in FIG. 2, the waveform of the detected signal S107 corresponding to the subbeam 107 shifts along the abscissa, i.e., radial direction from the waveform of the detected signal S106.
The amplitude of the tracking error signal "G" is maximized when the detected signals S106 and S107 are apart from each other along the abscissa by a half of the track pitch Tp, that is, when the distance between the subbeams S106 and S107 in the radial direction "P" of the optical disk file is half the track pitch Tp.
In view of the above, the pitch and rotational angle of the diffraction grating 51 are determined such that the main beam 105 is located at the midpoint between the subbeams 106 and 107 and such that each of the subbeams 106 and 107 is offset from the main beam 105 by a quarter (1/4) of the track pitch Tp in the radial direction P of the optical disk file 70.
In a conventional optical disk file 70, such as a read-only compact disk or a rewritable magneto-optical disk, a track pitch is approximately 1.6 .mu.m. According to the example mentioned above, since the distance between the main beam 105 and each of the subbeams 106 and 107 is set at 0.4 .mu.m in the radial direction, the amplitude of a tracking error signal is maximized to facilitate the tracking operation.
In the conventional head tracking system, a beam is emitted from the light source 101 and is then passed through the collimating lens 102 to be converted into a parallel beam. The parallel beam is diffracted by the diffraction grating 51 such that at least a pair of first-order diffracted beams are generated. The diffracted beams first pass through the beam splitter 52 and then through the objective lens 103, thereby focusing onto the optical disk file 70.
The main beam 105 and the subbeams 106 and 107 fall on and are reflected from the optical disk file 70. These reflected beams fall on the photodetector 54 via the objective lens 103, beam splitter 52, and refocusing lens 53, and are detected by the detecting sections 55, 56, and 57 of the photodetector 54. The detected signal S105 is output to another signal processing system of the optical disk drive and is processed therein as a reproducted signal or a focused error signal. The detected signals S106 and S107 are supplied to the tracking error calculating section 58, which generates the tracking error signal "G" based on the signals S106 and S107 and outputs the same to a control system of the optical disk drive. The control system controls the optical head in accordance with the tracking error signal "G" so as to perform the tracking operation.
There has recently been proposed an optical disk drive which uses a digital video disk (DVD), i.e., a high-density disk having a larger capacity, in addition to an ordinary disk such as a compact disk. Various types of high-density disks of this type are proposed, and their track pitches are approximately 0.8 .mu.m, about half that of the ordinary disk.
In a conventional head tracking system, the locations of the three beams is adjusted beforehand by a diffraction grating so as to optimize the distance between the beams for the track pitch of an ordinary disk. As a result, when a high-density disk is used which has a track pitch about half that of the ordinary disk, a satisfactory tracking error signal cannot be obtained.
In this case, the diffraction grating may be adjusted again in order to obtain an optimum tracking error signal. However, the adjustment hinders users from operating the disk drive for the ordinary disk.